1. Field of the Invention
The invention relates to sacrificial cathode-type electric arc vapor deposition.
2. Description of Related Art
The technique of electric arc physical vapor deposition is not new. The elements present in such apparatuses can be generalized to include an electrically biased coating source material serving as a cathode, an anode positioned apart from the cathode, and an initiator to initiate the arc discharge. The arc provides the energy for vaporizing the solid source material constituting the cathode. The vaporization from the cathode surface is very localized, and the high energy point at which the vaporization is occurring is often called the "cathode spot." The discharge is carried out in a chamber maintained at negative pressure. The vaporized source material or plasma, includes atoms of the source material, ions of such atoms, molecules containing atoms of the source material, and ions of such molecules. The process of depositing such plasma particles onto the substrate can be enhanced by creating an ionic attraction between the plasma particles and the substrate surfaces. Typically, this is accomplished by applying a bias voltage to the substrate.
Several techniques are known for coating by physical vapor deposition using the vacuum arc discharge. The various techniques can be differentiated by their attempts to improve design performances and coating process parameters. Historically, the first attempts employed only the natural physical properties of an arc discharge plasma produced by cathode spots for a coating deposition. The design efforts focused on stabilization of the discharge on the cathode surface, improving cathode material utilization, and improving the reliability of the arc ignition. Stabilization was usually achieved using some magnetic, electrostatic, or physical method for controlled movement of the cathode spot. The resulting plasma stream generated by the cathode spots diffused during propagation. Plasma density naturally decreased with the square of the distance between the cathode and the substrate. The magnetic fields that were used to control cathode spot movement were not strong enough to change substantially the plasma flow parameters.
U.S. Pat. No. 3,625,848 describes a beam gun for use in creating an arc discharge in which electrodes are positioned at different locations to increase the deposition rate. U.S. Pat. No. 3,793,179 describes an apparatus in which random motion of arc spots on the cathode surface are extinguished by a shield when the arc spot moves off the desired evaporative cathode surface.
The cathode electrode can be made of different electroconductive materials and may have different shapes and positions relative to the anode electrodes. The process vacuum chamber is often used as an anode or as a portion of the anode electrode assembly. The anode is sometimes positioned within the vacuum chamber, preferably along the magnetic force lines that are crossing the cathode surface to minimize the arc voltage and provide a certain stability of the arc discharge. The arc discharge is usually powered by low voltage welding power supplies. Such power supplies were used to sustain arc discharges of low voltage to prevent cascade arcing. Such discharge voltages were close to the cathode voltage drop. As a result, there were not significant electric fields existing in the plasma.
An arc discharge can be ignited by injection of plasma into the gap between the discharge electrodes. Ignition has been accomplished by means of electromechanical or pneumatic-driven current disconnectors or by high voltage pulsed devices. Reliability of the ignition devices depends on the geometry of the discharge electrodes and other, mostly empirical parameters. Positioning of the igniter can also be critical in determining the probability that ignition will occur. Igniter positioning is important because arc spots originate from the igniter and can severely damage the igniter assembly. It has been discovered that exposure of the small metallized ceramic elements of the high voltage pulsed igniter device to the arc discharge current while it is positioned in direct electrical and mechanical contact with the cathode electrode is common and problematic. Such positioning decreases the productivity and reliability of the apparatus and results in unnecessary expense due to the need to remove the igniter before replacing the consumable cathode.
Another area of work has focused on improving the quality of the plasma flow generated by the cathode spots through control of the movement of the cathode spots by external static or dynamic magnetic fields. Movement of the cathode spots would have the effect of reducing the overheating that occurs around static cathode spots and decreasing the droplet, or macroparticle, contamination of the plasma flow. Different designs have created a magnetic field whose tangential component is located just over the cathode surface. The magnetic field is then used to steer the motion of the cathode spots in the -j.times.B or "retrograde" direction, seemingly contrary to Ampere's law. Another approach to the problem has involved use of the internal magnetic fields created by non-uniform arc current distributions in an extended cathode body, a flat rectangular plate, or an elongated rectangular bar or cylinder. These internal magnetic fields interact with an internal magnetic field of the arc discharge column or a small external magnetic field to allow a plasma generation area over the surface of these extended cathodes, thereby improving process productivity and coating uniformity on the substrate. This approach, however, involves the use of magnetic fields which are localized near the cathode surface. These magnetic fields are of such shape and spatial distributions that they cannot change substantially the plasma energy characteristics in the volume. An increase in magnetic field strength in such designs in which the magnetic field means is below the cathode working surface causes arc discharge destabilization or extinction.
Various cathode shapes have been investigated in the prior art. U.S. Pat Nos. 4,609,564, 4,859,489, and 5,037,522 describe the use of a long cylindrical cathode or rod which uses the magnetic field of the arc current itself to induce motion of the arc along the cathode surface. The systems employing flat planar and cylindrical cathodes without external magnetic fields are not stable enough to permit increases in arc current without a tendency of the plasma column to be pinched by the forces of an internal magnetic field of the discharge current and without causing damage by localized melting of the cathode and adjacent elements of the apparatus. When such systems are operated at low arc currents the associated process productivity and flow plasma characteristics are considerably reduced.
The need to induce movement of the arc spots on the cathode surface for the purpose of uniformly consuming the cathode has also been addressed in the prior art. U.S. Pat Nos. 4,673,477 and 4,849,088 induce cathode spot movement through mechanical movement of magnets generating the fields which move the arc spots. U.S. Pat. No. 4,724,058 accomplishes this by use of multiple electromagnets. Cathode size and geometry have also been used to accomplish this arc spot movement and more uniform cathode consumption. German Democratic Republic Pat. No. 265,506 claims the use of axially symmetric magnetic fields parallel to the working surface of a cylindrical or conical cathode. These parallel fields can be created either by the use of oppositely polarized Helmholtz coils arranged above and below the working surface or a toroidal coil configured coaxial to the cathode at the height of the working surface. Helmholtz coils are known to produce a highly homogeneous magnetic field in a large volume of space. They consist of a pair of equal strength and similarly polarized, i.e., parallel co-axial magnetic coils connected in series which are separated from one another by their mean radius. If the Helmholtz coils are polarized oppositely, i.e., in an anti-parallel fashion, the radial magnetic field between the coils will be substantially less than the axial component of the magnetic field inside the coils because the flux created by the currents in both coils has passed through the openings of the coils and must pass also through the radial slot between the coils. A ratio of cross sectional areas of the coil openings and the radial slot between the coils determines the ratio of the axial and the radial components of the magnetic field strength. The invention is characterized by a cross sectional area of the radial slot between the coils that substantially exceeds the area of the coil openings, and this results in a small radial magnetic field component of only about 0.1-1.0 milli Teslas near the cathode surface, which is not sufficient to effectively control cathode spot motion.
Many attempts have been made to channel and improve the quality of the plasma stream during propagation between the cathode and the substrate surfaces. The plasma stream generated by cathode spots diffuses as it moves towards the substrate, and the plasma density naturally decreases with the square of this distance. The beam pattern of the plasma stream is slightly different from that predicted by the cosine law because of the combination of neutral and ion components. An increase in the arc current increases this difference due to the focusing action of an internal magnetic field produced by this arc discharge current. Application of an external longitudinal magnetic field focuses the plasma components of the plasma stream and improves, to some extent, the cathode material utilization. The plasma characteristics, however, remain the same as they would be without a magnetic field because the magnetic fields applied are not strong enough to change substantially the ionization processes in the volume.
Stronger magnetic fields have been applied to separate the plasma component from the macroparticles by deflection of the plasma through the use of curvilinear and/or rectilinear magnetic filters. These filters are typically a combination of solenoids which guide the plasma component along the magnetic force lines and a set of baffles along the chamber walls to collect the relatively massive macroparticles. Use of such a curvilinear magnetic filter approach can improve the coating quality substantially. However, this approach is difficult to apply when coating using relatively low melting-, or multicomponent-cathode electrodes, where the degree of ionization of the plasma is low or may differ substantially for the different components of the alloy. In these cases, coating deposition rates drop substantially from the conventional cathodic arc value or change the composition of the coating from that of the alloy. As a result, this filtering technique may be only of academic interest.
A completely different approach was proposed in the late 1970's to improve the arc coating technique. The technique involved the use of an externally applied magnetic field to accelerate the plasma using the principle of the Hall plasma accelerator. A steady state Hall accelerator is described as a co-axial system of electrodes including a butt-end central cathode electrode made of the material to be transformed into the plasma state by vacuum arc discharge from the cathode spots and a copper anode shaped as a cone nozzle positioned outside the cathode electrode. A gas injecting distributor positioned close to the substrate is described and used for injecting a working or ionizable gas into the process chamber. The plasma acceleration was carried out using an internal azimuthal magnetic field created by a discharge current in excess of 1000 Amps or in an externally imposed axially diverging magnetic field. In the latter case the accelerator is provided with a magnetic coil on the anode that creates a magnetic field distribution of a shape that looks similar to that described much later in U.S. Pat. No. 5,126,030. An electrically insulated screen and the magnetic field created by the coil provides stabilization of the cathode spots on the central part of the cathode flat surface, and the plasma stream is focused along the accelerator axis. When the magnetic field is applied a general rotation of the plasma can be observed. This rotation is believed to be due to a revolving pattern of the cathode spots over the surface of the cathode. An increase of the magnetic field strength to over 50 Gauss, however, causes an overheating of the cathode and a corresponding increase in the droplet or macroparticle component of the plasma flow. These effects limit the performance of the accelerator in the useful magnetic field intensity range. At field intensities less than 50 Gauss, a certain separation of the droplet component of the plasma flow was described as the magnetic field was increased. This was explained as being due to the charging of the droplets by collision with plasma electrons followed by an electric field action that pushes the droplets out of the plasma stream.
U.S. Pat. No. 5,126,030 issued to Tamagaki et al. describes a cathodic arc deposition method and apparatus comprising at least one magnetic coil means to produce a magnetic field arranged between the arc evaporation source and the substrate. The magnetic field lines are constricted in the space between the evaporation source and the substrate. The vacuum chamber, acting as the anode, is connected to the positive pole of the power supply. The coating deposition rate increases up to about the "saturation value" of 0.25 .mu.m/min as the magnetic current is increased. The increase of plasma density and the existence of a comparatively low deposition rate at the "saturation" level at such a short distance from the coil can only be explained by the focusing action of the magnetic field on the plasma stream generated by the arc discharge. Such focusing does not increase the productivity, or deposition rate, above that which would be expected from an apparatus without these features, and the focusing does not increase the quality of the coatings. In fact, such focusing might even be expected to result in considerable non-uniformity of coating thickness across the substrate surface.
U.S. Pat. No. 5,458,754 of Sathrum et al. discloses a plasma enhancement apparatus and method for coating deposition. A tubular plasma guide is connected to the face of the cathode with a solenoid arranged around the non-magnetic stainless steel duct. The magnetic field created by the solenoid is purposely non-homogeneous to create a strong and confined "bottleneck" magnetic field within the plasma guide. The geometrical factors influencing the magnetic field orientation and gradient include the number and the separation distance of the air gap. An independent direct current up to 70 Amps through the solenoid can produce magnetic fields as high as 150 mT. With the design described, the magnetic field maximum intensity is located axially just in front, that is, to the substrate facing side, of the consumable cathode. An increased velocity of arc spot motion is achieved by magnetic pole pieces housed inside the coil to create a transverse component of the magnetic field in the vicinity of the cathode face. The necessity to increase the axial component of the magnetic field up to 150 mT causes extreme compression of the plasma flow. This fact, combined with the high magnetic field strength required for the cathode spot motion control simultaneous with the need for stable arc discharge operation limits the cathode dimension and, therefore, the coating productivity of the apparatus. The observed decrease in droplet concentration in the plasma stream is mainly due to the high magnetic field which causes constriction of the stream and the physical dimensions of the outlet aperture. Such a geometry results in extremely high non-uniformity of the coating thickness on the substrate, in turn limiting the industrial applicability of the apparatus.
Semenyuk has described a discharge system which uses a water-cooled co-axial cathode and anode. The side surfaces of the cathode are bordered by a single screening electrode which is not in electrical contact with the other electrodes of the discharge system. The external magnetic field is created by two magnetic coils located above and below the active cathode surface. The coils are energized in contrary. The arc discharge is initiated by injection of the plasma "clot," or portion of plasma, and appears when the impulse discharge occurs over the surface of the metal coated ceramic bar placed between the cathode and the screen electrode. A plasma flow was formed using these conditions with an angle of divergence of 20 degrees. An increase in the external magnetic field up to 125-250 Gauss causes ion current densities along the plasma stream axis to increase by about five times. However, by using a magnetic field of 125 Gauss the ion current density in the radial direction decreases from 10 mA/cm.sup.2 at the axis to 5 mA/cm.sup.2 at a radius of only 5 cm. This spatial non-uniformity leads to undesired coating non-uniformity. The high magnetic fields required to make this apparatus work limits cathode surface utilization and adds to non-uniformity because the area over which the arc can exist is limited by induction. The design described causes at least a 1.5-fold increase in discharge voltage, which in turn causes cascade arcing and damage to the screen electrode, electrical insulators, vacuum seals, and other elements, making the design less commercially practical. The positioning of the metallized ceramic "igniter" between the cathode and the screen electrode exposes the igniter to cathode spot action. Operation of the device in this manner continually severely damages the igniter device thereby preventing efficient operation of the arc discharge device.
It does not appear to be recognized in the prior art that geometrical design factors for the apparatus described above could result in sub-optimal performance. I have observed that improper positioning of the entire magnetic field configuration relative to the positioning of the cathode and the anode can result in dramatically sub-optimal coating deposition rates in the case where plasma ions evaporated from the cathode surface are trapped inside the apparatus for reasons that will become clear in light of the disclosure below.
Some applications of the cusp-shaped magnetic field are known in the prior art. U.S. Pat. No. 4,952,843 of 1990 of Brown et al. describes an ion source in which a multi-pole cusp magnetic field produced by a number of a permanent magnets arranged in the vacuum chamber 4, where the plasma is expanded, is used to flatten the ion current flow generated by this source. An article by Askenov, "Formation of Filtered Intent Vacuum-Arc Plasma Flows," XVII.sup.th International Symposium on Discharges and Electrical Insulation in Vacuum, Berkeley, Calif., Vol. 1,895-899, 1996, describes an apparatus in which an axially symmetric magnetic field of an acute angle configuration was used for filtered coating deposition. This cusp magnetic field configuration is arranged inside the vacuum chamber between two axially cathodic arc discharge devices so that plasma of the cathode material generated there flows into the cusp through the axial cusp "openings," is treated in some fashion inside the cusp, and thereafter flows out of the cusp through the radial (ring) slot of the cusp to the substrate being coated.
There are instances of prior art disclosures in which the magnetic field strength has been intentionally varied at different points along the central axis of line-of-sight type cathode arc devices. U.S. Pat. No. 4,551,221 to Axenov et al. ("Axenov '221") describes a cathodic arc vapor deposition device having a single solenoid disposed co-axially along the central axis at one end of which is the consumable cathode and the substrate at the other end. The solenoid extends from a point behind the cathode working surface and ceramic jumper of the igniter to a point very near the substrate. The "front" cathode surface faces the substrate. The number of turns per unit length of the solenoid adjacent the cathode behind the working surface is twice that of the rest of the solenoid. Such arrangement produces a magnetic field that is stronger in the vicinity of the cathode behind the working surface and weaker between the working surface and the substrate. According to the patent, with this arrangement the plasma flow generated by the consumable cathode is injected into a longitudinally axially symmetrical magnetic field with a "plug" behind the working end surface of the cathode. The equipotential surfaces converging behind the working surface reflect the ion component of the plasma towards the target. According to the patent, such arrangement of the solenoid provides "optimum conditions required for stable ignition and maintenance of the arc discharge" (emphasis supplied). As will be apparent in light of the following disclosure, the Axenov '221 patent teaches away from the magnetic field arrangement of the instant invention. In the disclosure which follows, the arc discharge is quite suitably stabilized even when the front magnetic field strength exceeds that of the field behind the cathode working surface.
Another example of non-uniform magnetic field strength along the central axis is described in U.S. Pat. No. 4,452,686 to Axenov et al. A single plasma-focussing solenoid encompasses the tubular section. The solenoid is connected in opposition to the coil of the magnet. The coil of the magnet is arranged in the tubular plasma guide on the axis thereof. The number of turns of that portion of the solenoid between the centrally disposed magnet coil and the substrate is higher than the number of turns of the portion closest the plasma source. The patent states this arrangement enables the most effective focusing of the outgoing plasma flow, results in higher density, and consequently increases the coating deposition rate. No portion of the solenoid appears to be positioned axially behind the working surface of the cathode.
U.S. Pat. No. 4,724,058 to Morrison, Jr. describes adjacent sections of magnetic fields having reverse polarity provided by adjacent linear lengths of a continuous closed loop conductor carrying current flowing in opposite directions. The conductors are positioned on the side of the target opposite the evaporative surface. The purpose of this arrangement is to control cathode spot movement.
While many commercial cathode arc systems employ cylindrical co-axially aligned cathodes, other cathode shapes are known. For example, U.S. Pat. No. 5,269,898 to Welty describes controlling the movement of the cathode spot along the surface of a cathode rod using a helical magnet coil coaxially aligned with the cathode. U.S. Pat. No. 5,480,527 to Welty describes a rectangular shaped cathode source.
While one goal of the various cathode spot control techniques described above has been reduction of macroparticle generation, others have attempted to filter or block the passage of the macroparticles once formed from depositing on the substrate. U.S. Pat. No. 4,511,593 to Brandolf, for example, describes a shield which blocks all line-of-sight plasma particles from depositing on the substrate. Undesired agglomerates are intercepted in this manner. Desired atoms, ions, and molecules are said to reach the substrate by diffusing around the shield. U.S. Pat No. 4,452,686 to Axenov et al. describes a tubular section of the vacuum chamber through which the plasma particles pass prior to deposition on the substrate. A series of ribs extending radially inwardly a short distance from the inner wall, preferably formed by rings arranged one after another in parallel planes perpendicular to the section axis, are arranged immediately adjacent the interior wall of this section. The ribs inhibit passage of macroparticles through the tubular section.
There remains a significant need in the electric arc vapor deposition art for improved plasma flow control in the generator to achieve higher value-added coated products at a competitive price.